Plan B refers to the auxiliary plan teams will turn to when Plan A has demonstrably failed. Generally speaking a team chasing a goal or needing to protect a lead will desperately resort be 'Plan B' in around the 85th minute, or earlier depending on the quality of the team they are facing.
The Need for a Plan B
Having a Plan B was seen as entirely necessary up until Barcelona's victory against Manchester United in the 2009 Champions League. Whereas before it was accepted that all teams need a back-up option when Plan A wasn't working, the ridiculous ease with which Barcelona dominated Spanish and European football between 2009 and 2011 convinced many that when Plan A is good enough, there is no need for a Plan B.
However, as Barcelona began to struggle to break teams down with their incessant passing towards the end of the 2010-2011 season, and as they ultimately lost the title to Mourinho's Real Madrid in the 2011-12 season, public opinion on the need for a coherent 'Plan B' has once again shifted. Many armchair football experts began to opine the lack of a Plan B was costing Barcelona, an assertion seemingly proved correct as they somehow managed to lose against Chelsea over two legs in the Champions League.
Plan B Tactics
Traditional Plan Bs
Plan B tactics are entirely dependent on whether a team is seeking a goal, or seeking to defend theirs. Offensively, Plan B tactics are traditionally largely in line with the tenets of no-nonsense and 'Route 1' football.
Defensively, teams may aim to 'park the bus' whilst urging players to use their experience to break up enemy attacks, and generally waste time, employing a 'big man' to sit on the halfway line to give defenders a target towards which to aim their no-nonsense clearances.
Alternative Plan Bs
Higher quality teams, and especially those with managers who like to see themselves as practioners of 'beautiful football' are less willing to resort to 'Route 1' football in attack. Such managers largely struggled to come up with an alternative until Arsene Wenger's revolutionary 'basically play the same but leave Thomas Vermaelan hovering around the 18 yard line' strategem. This tactic has proven popular and has been adapted by other managers who are too proud to have any big strikers in their squads. Recentl examples include Spain in the 2010 World Cup and Harry Redknapp's Tottenham in the 2010-11 season employing Gerard Pique and Younes Kaboul in the Vermaelan role respectively (although there has been speculation that Kaboul simply joined the attack and was too tired to track back).
Use of Plan B as Plan A
Despite the shift towards a more technical style of play from the long ball tactics of yesteryear, there are still teams in the EPL who incorporate tactics officially designated as Plan B into their Plan A. This is largely frowned upon, but seems to be a tried-and-tested method of attaining about 15th (see: Stoke).